When I was wet behind the ears in my hydraulic career, I was confused by the myriad pressure valves available to hydraulics. I came from a car wash background, where unloading valves are used to regulate the pressure of a piston pump. On the day in class where we discussed pressure valves, I had a hard time differentiating a relief valve from the carwash unloading valve that I was used to. On top of relief valves, I was introduced to pressure reducing valves, sequence valves, counterbalance valves and crossover relief valves, to name a few—furthering my initial confusion.
Most of these pressure valves operate like a relief valve, in that when pressure at their inlet port reaches a pre-set value, it starts to open. The only real differences between them are secondary functions, like the presence of a check valve, a drain in the spring chamber, or maybe existence of a pilot port. However, each and every pressure valve is normally closed, meaning the valve remains closes until pressure reaches that pre-set valve I mentioned, at which point the valve opens to allow flow.
The exception to the above rule is the pressure reducing valve. It is the only pressure valve that is normally open, meaning that it flows all day long until a pre-set pressure is reached. When downstream pressure reaches a level strong enough to counteract the force of the spring, it starts to close. The result is that flow is no longer stuffing the actuator, and pressure immediately decays. The response is rapid, smooth and unnoticeable; pressure simply maintains itself at the pre-set pressure.
Pressure reducing valves are happiest in hydraulic circuits with variable displacement pumps. This is because when a pressure reducing valve closes to control downstream pressure, the fluid going into the valve needs to go someone. The incompressible nature of fluid means it must go over the relief valve should you be using a fixed displacement pump. Although no real harm will be done, it is not the most efficient option, as the excess fluid passing across the relief valve is wasted as pure heat. A pressure compensated pump will simply reduce flow to meet the lower demand created as the reducing valve is closed.
“Reducing” valves are available as stand-alone cartridge valves, sandwich valves and inline valves, amongst others. The higher performing version will have a relieving function so that downstream pressure in a stalled actuator can be drained to tank. Reducing valves are used in any system where a single actuator or other sub-circuit requires pressure lower than that of primary system pressure.